A dwarf seahorse can sneak unnoticed remarkably close to prey — less than a penny’s thickness away — thanks to the way the horsey head shape moves through water.

If they had just two or three milliseconds of warning, some of the seahorse’s prime prey, tiny crustaceans called copepods, would scoot away, says Brad Gemmell of the University of Texas at Austin. But a dwarf seahorse (Hippocampus zosterae) can strike even faster, within one millisecond. To succeed with its speedy strike, the seahors...